LESSONS IN ASTRONOMY. 



213 



The origin of tho Royal Observatory and of this pott waa in 

 the year 1675. Great inconvenience had bean experienced in 

 long Toy ages from the want of- some method of determining 

 the longitude in which a vessel was at any time, bat at length a 

 plan was proposed which was substantially the same M one in 

 use at the present time. This consisted in noticing very accu- 

 rately tho position of the moon with respect to neighbouring 

 fixed stars. As the earth moves in its path, this position seems 

 to vary. If then we have an accurate list of these " lunar dis- 

 tances," as they are termed, calculated for any given meridian of 

 longitude, wo shall be able to tell by observation what the time 

 is at that meridian. We can then compare this with the local 

 time of the place where we are, and in this way ascertain the 

 longitude ; for since 15 of longitude make a difference of one 

 hour in the time, we have only to allow 15 for every hour of 

 difference in the times, and we shall at once tell the longitude. 



The method of solving this problem usually employed now is 

 merely to compare a good chronometer, set to Greenwich time, 

 with the local time ; but it was not till a comparatively recent 

 period that chronometers were made accurate enough for this 

 purpose, and even now it is a great advantage to be able occa- 

 sionally to check them by means of lunar observations. 



When this plan of ascertaining longitudes was proposed, an 

 objection was taken to it on the ground that the tables of 

 the positions of the moon and fixed stars which then existed 

 were not sufficiently accurate to be of any practical use for this 

 purpose. 



It was decided, therefore, that an observatory should be 

 built and sustained with this especial end in view, and Flam- 

 steed was appointed Astronomer Royal. An elevated position 

 in Greenwich Park was accordingly selected as a suitable site for 

 this observatory, and here it was erected in 1675, and from that 

 time to the present some of the greatest astronomers have 

 resided in it, and an almost uninterrupted series of observations 

 has been maintained. These have constantly proved in many 

 different ways to be of the greatest practical utility. One main 

 duty connected with the Observatory is the preparation of the 

 " Nautical Almanack." This is an almanack published three or 

 four years in advance, and containing a large number of impor- 

 tant astronomical tables. The position of the moon with respect 

 to many of the fixed stars is shown for every third hour 

 throughout the year. The position of the various planets is 

 also shown, as well as the eclipses and occnltations of Jupiter's 

 satellites, and many similar tables, which are useful to the 

 mariner in ascertaining his position, as well as to the astronomer. 

 The reason of its early publication is in order that captains 

 about to set sail on long voyages may take it with them. 



Though the Observatory was thus founded by Government, it 

 was some time before it was provided with instruments worthy 

 of the place, Flamsteed having to use his own for a considerable 

 period. No very important discoveries are associated with the 

 name of this astronomer, but he was a very careful observer ; 

 and it appears that it was to his accurate observations that 

 Newton was greatly indebted in many of his inquiries. 



Halley succeeded Flamsteed in his duties at the Observatory. 

 He was for some time an intimate friend of Newton, and made 

 several long journeys in the interests of science. An expedition 

 was fitted out under his charge to observe and catalogue those 

 stars in the southern hemisphere which are invisible in this 

 country, and a list of nearly 400 was compiled. This, however, 

 was by no means a complete one, as the station chosen for 

 observation (St. Helena) was in many respects unfavourable. 



After the discovery which Newton had made, that bodies 

 under the joint influence of an original impulse, and the attrac- 

 tion of a central body, might revolve in an hyperbola or para- 

 bola as well as in an ellipse, the appearance of a comet was 

 anxiously awaited, in order that if possible it might be ascer- 

 tained whether these bodies moved in fixed orbits of either of 

 these forms, or whether they were merely stray wanderers 

 dashing swiftly past our system, and then for ever lost in the 

 deep abysses of space. 



In the year 1680 this desire was gratified by the appearance 

 of a very large and remarkable comet, which attracted great 

 attention both by its brilliancy and the rapidity with which it 

 travelled. Halley devoted his earnest attention to the study 

 of this body ; its motion was accurately noticed and recorded 

 by him, and he discovered that a parabolic orbit could be con- 

 structed which would account for all its movements. Its 



eccentricity, however, WM so great that a period of about 

 600 yean most elapaa before it coold again return to the inn. 



After thia comet had pawed away, Halley rial! devoted hia 

 attention to the subject, carefully inquiring into the recorded 

 appearances of different comet*, with the view of aaoerUuning 



whether the intervals between the appearances of any of the moat 

 noticeable ones appeared in any way uniform. Shortly after 

 this, in the year 1682, a second large comet appeared, and now, 

 with tho information he ha*, already acquired, Hallry waa in a> 

 bettor position to inquire into its motion. He accordingly did 

 this, and after a time announced that he had calculated tta 

 orbit, and found that it moved in an ellipse, ita aphelion dia- 

 tanoo being nearly 3,500,000,000 miles; also, that it* 

 waa about seventy-five yean. He then looked back through 

 his list of comets, and found that be could distinctly trace 

 it back for a considerable period. This so far continued hit 

 former calculations that he distinctly foretold ita reappearance 

 about the close of the year 1758; and so convinced waa he of 

 the truth of this prediction that he requested, since he could 

 not live to witness its return, that, when it waa fulfilled, people 

 might remember it was an Englishman who had first traced the 

 path and prophesied the return of a comet. 



Long before the date assigned for the return of thia comet, 

 which was known as Halley's, he himself had paaeed away. 

 Astronomers were, however, on the watch, and some French 

 astronomers in particular investigated most carefully and in* 

 dustrionsly the retarding effect which would be produced on the 

 comet by the attraction of the planets, and as a result of 

 their inquiries, announced that it would be slightly delayed 

 by the action of Saturn and Jupiter, so that its passage round. 

 the sun might be expected on the 13th of April, 1759. Just at 

 the close of the previous year a wanderer was detected by an 

 amateur, and as it came nearer it proved to be the very one 

 whose return hod been for so long a time foretold ; and 

 though its period of revolution was upwards of three-quarters 

 of a century, yet the observations and calculations were so 

 accurate that it actually passed the sun within less than 

 three weeks of the predicted day. 



On the occasion of the next return of this comet, which was 

 in 1835, not only was the date, but the place of its appearance, 

 pointed out ; and on turning a large telescope to that spot, the 

 comet was seen as a faint cloudy object. We see thus that 

 Halley's comet may now be reckoned as one of the members of 

 our system whose motions are fully understood. Its next return- 

 may be expected in the year 1912. 



Bradley was the next Astronomer Royal. The great dis- 

 covery which has rendered his name memorable is that of the- 

 aberration of light, a phenomenon the explanation of which- 

 must be deferred for the present. He also took an active part 

 in the reform of the calendar, which had by this time varied 

 considerably from the true seasons, and in order to rectify the- 

 error, joined in recommending that eleven days should be 

 struck out of the month of September, 1 752, so that the day 

 following the 3rd of that month was called the 14th. Thia 

 measure was very unpopular at the time, and Bradley came in 

 for a large share of the popular dislike on this account ; and hi* 

 death, which occurred a few years afterwards, was by many of 

 the ignorant regarded as a mark of Divine displeasure at hia 

 presumption in thus daring to interfere with the regular order 

 of the calendar. 



This alteration has since been made in nearly all countries 

 except Russia, where dates are still reckoned according to the 

 old style, and are now thirteen days behind those used in the 

 rest of Europe. 



In the year 1778 Professor Bode, of Berlin, published a very 

 remarkable law relating to the distances of the planets from 

 the sun, which, though it is said to have been discovered by 

 Titius, is always known as " Bodo's Law." It was at first 

 merely a bold conjecture, but has since attracted much atten- 

 tion, as it partly led to the discovery of the first of tho minor 

 planets or asteroids. 



He observed that if wo take the numbers 



II 



M 



each of which (after the second) is double that which precede! 

 it. and add the number 4 to each of them, we obtain the follow* 

 ing list, which represents approximately the distances of the 

 planets named under them . 



